1. Field of the Invention
The present invention relates generally to a method and apparatus for injecting a liquid sample into a gas chromatograph, and more particularly to the on-column injection of such a sample into a capillary column.
2. Description of the Prior Art
Gas chromotography provides for the separation and quantitative determination of gases, volatile liquids, and solids. The separation is carried out in a column which comprises the fixed or immobile phase. The immobile phase may be a column packing, such an as inert solid support coated with a non-volatile stationary liquid. The stationary liquid is chosen to be non-volatile at the column temperature and to provide the proper interaction with the materials being selected. For example, relatively polar liquid phases are chosen for the selection of polar solutes, while non-polar liquid phases are used to separate relatively non-polar solutes. More recently, very high efficiency separation has been achieved with glass, typically fused silica, capillary tube columns having inside diameters of from about 0.007 to 0.020 inches where the stationary liquid phase is coated directly on the inside of the capillary tube.
Chromatographic separation is carried out by injecting the sample to be analyzed into a carrier gas stream flowing through the column. The solutes are selectively adsorbed into and evaporated from the stationary phase, resulting in a differential rate of migration through the column for different species. The species are identified by their characteristic transit time through the column. A detector, such as a thermal conductivity detector, a flame ionization detector, or a spectrometer, is located at the outlet of the column for detecting the emergence of the species over time.
In the case of gas chromatographic analysis of liquid samples, it is necessary to maintain the column at an elevated temperature (so that the liquid sample exhibits sufficient vapor pressure to pass through the column by repeatedly and alternately diffusing into the gas phase and dissolving in the liquid phase). A variety of techniques may be used for vaporization of the liquid sample prior to, or simultaneous with, injection into the column. Conventionally, a pre-heater, or injection-port flash vaporizer, heats the sample to a temperature so that the sample vaporizes rapidly when injected into a stream of heated carrier gas. When the major part of the sample volume which has been vaporized in this manner is directed to the column, the technique is referred to as "splitless" injection. Frequently, because of the limited capability of capillary columns to handle high volume sample concentrations (such as with neat or undiluted materials), it is necessary to inject only a portion of the vaporized sample into the column. This is known as split injection. Such split injection can result in a discriminatory separation of the components in the sample upon vaporization, causing inaccurate determination.
To allow the direct injection of small sample sizes, on the order of 0.1 to 2 .mu.l, various on-column injection techniques have been developed. Much of the early work in this field was done by Grob and Grob, Jr. and is reported in a series of articles. See, e.g., Grob et al. (1974) J. Chromatography 94:53-64; Grob et al. (1978) J. Resol. Chromatography 1:57-64; Grob et al. (1978) J. Chromatography 151:311-320; Grob (1978) J. Resol. Chromatography 1:263-267; and Grob et al. (1979) J. Resol. Chromatography 2:109-117.
The on-column injection method of Grob and his coworkers utilizes a lengthy, narrow syringe needle which allows injection of the samples into the column at a point within the chromatograph oven. In this way, the sample can be vaporized directly as it is injected into the column. Such direct injection of a liquid into a column heated above the solvent boiling point, however, can create problems. First, the solvent will rapidly vaporize causing the more volatile components to be lost as they are pushed by the pressure pulse backward past the needle and out of the column. Moreover, less volatile components may remain in the needle because of the pre-volatilization of the solvent and its rapid loss from the needle. Even if the user attempts to inject the sample slowly in an attempt to minimize the pressure pulse caused by the temperature gradient, capillary forces will draw the sample along the needle resulting in the loss of less volatile components. See Galli et al. (1981) J. Chromatography 203:193-2.5. Finally, the distributed volatilization of the sample over the period of the injection causes band broadening since those portions of the sample which are first injected have a lead over the remaining portions in reaching the detector end of the column.
In an attempt to overcome these problems, it has been proposed to cool the initial length of the chromatographic column in the oven by directing a stream of cooled air alongside. This technique is described in U.S. Pat. No. 4,269,608 to Sisti et al. Such secondary cooling, however, is not completely effective due to the low heat capacity of the coolant, the rapid mixing of the cooled air within the heated oven, and the low thermal mass of the fused silica columns. Moreover, such cooling can result in a loss of component resolution because the moving sample must ascend a positive temperature ramp.
Thus, it would be desirable to provide an improved method and device for injecting liquid and other samples into capillary columns used in gas chromatography.